22 ME. G. M. DATIES ON THE [vol. lxxV, 



YII. Petrological Examination or the Deposits. With 

 Chemical Analysis by A. Broughton Edge, M.B.E., F.C.S. 



The deposits will be described in the order in which they now 

 occur in the pipes : namely, Chalk, Grey Clay- with- Flints, Red 

 Clay-with-Flints, sand, and pebbles, although that is not the order 

 of age. The occurrence of halloysite will then be dealt with, and, 

 in conclusion, the mode of origin and subsequent history of the 

 deposits will be considered. 



Chalk. 



The chalk belongs to the zone of Micraster cor-testudinariiim, 

 and contains flint-nodules scattered in moderate amount along 

 the bedding-planes. 



In places the chalk is abnormally hard, apparently through 

 deposition in its pore-spaces of carbonate of lime dissolved from 

 higher layers of chalk. Thin sections of this hard chalk show no 

 reciystallization of calcite or other peculiarities of texture. A 

 sample treated with dilute hydrochloric acid gave a residue of 

 099 per cent, brown clay, and 0011 per cent, coarser material, 

 the latter being chiefly quartz with a little flint, silicified forami- 

 nifera, ferruginous matter, etc. A feature observed in rubbing 

 down the sections was the number of dendritic patches of man- 

 ganese oxide, about 05 mm. in diameter, scattered through the 

 chalk and not concentrated along the joints as is often the case. 

 They appear to represent primary segregations of the manganese 

 diffused through the Chalk ooze, and originally extracted from the 

 sea-water by organisms or derived from basic igneous material. 1 



Adjacent to the pipes, the chalk is often soft and plastic 

 through partial solution. This material is easily washed down, and 

 yields abundant foraminifera, polyzoa, sponge-spicules, ostracods, 

 Inoceramns fragments, etc. Minute bones, teeth, and scales of 

 fishes, in brown translucent phosphate of lime, and small phos- 

 phatic nodules (l'0x0'5 mm.), also occur. Most of the foramini- 

 fera and some other fossils are silicified, the delicate tests having 

 been dissolved when not so preserved. The chalk, too, is partly 

 silicified, giving rise to porous masses which resemble the cortex of 

 flints, but without the black interior. 



Grey Clay-with-Flints. 



This is a dark-grey clay, almost black when wet, containing 

 numerous unworn Chalk flints. The flints often have black man- 

 ganese coatings, and occasionally groups of green-coated flints may 

 be seen, representing the remains of a bullhead bed which covered 



1 See J. Murray & A. Renard, ' Challenger Reports : Deep-Sea Deposits' 

 1891, pp. 372-78 



